In analog circuitry, a frequency mixer is also known as a “mixer,” “converter,” “detector,” “multiplier,” and sometimes “modulator.” For the purpose of this document, all those terms are interchangeable, and the general term “mixer” shall equate to all variations in terminology. Mixer circuits are characterized by multiple (usually two) input ports at differing frequencies and at least one output port at which appears at least the sum and difference of the two inputs, and can also include the sums and differences of harmonics of the two inputs, potentially creating a complex waveform. Those many frequencies appearing in the output of the mixer are typically applied to the input of a filter of appropriate selectivity (narrow bandpass) to permit selection of the frequency(ies) most useful in the overall design, and suppression of other frequencies.
In most mixer circuits, at least one of the input frequencies is variable, or controllable, by either automated or manual means. Such control determines the output of the mixer circuit. That controllable input is usually called the local oscillator, or LO.
One problem with existing mixers is that they use nonlinear components that create distortion of the output signal, which limits the performance of the overall system.
Typical mixers are comprised of a nonlinear component or circuit to which the signal frequency and the LO frequency are simultaneously applied. The result is a form of intermodulation, as multiple signals are created when the signals of the two frequencies interact with each other. Intermodulation products can be very complex, but it will be the subsequent filter at the output port—usually a bandpass filter—that extracts the desired signal and suppresses others.
Among the simplest nonlinear components used in mixers is a diode, which produces the original frequencies as well as their sum and their difference, along with many other mixing products. Complex mixers are comprised of many individual components, and provide the circuit designer with multiple options. But simple and complex mixers use nonlinear components which, by definition, produce output characteristics that reduce the performance of the overall system.
In a typical radio circuit, the output of the antenna will usually contain many carrier frequencies generated by multiple signals received from multiple transmitters. Early radio designers learned that it is difficult to manage and manipulate such a complex signal, but relatively easier to convert the desired signal to a standardized carrier frequency that is manageable, enabling the subsequent circuitry to be optimized for performance at that single frequency. In radios, that standardized frequency is usually called the Intermediate Frequency (IF). In the mixer, those multiple frequencies are combined with a controllable signal, the LO, which is varied to control the output of the mixer to generate that IF standard, among other mixer products. If a selective filter follows the mixer, then controlling the LO can shift the desired carrier frequency to that which will pass the selective filter, the output of which then becomes the IF, the signal to which the rest of the system has been tuned. That tuning circuit is usually followed by various forms of demodulation (extraction of a carrier's content) and amplification.
In a simple amplitude modulation (AM) radio, the antenna simultaneously receives many broadcast stations (frequencies), but the combination of an LO, a mixer, and a filter permits the user to tune the system to the desired station.
Obviously, the mixer—the frequency converter—is a critical component of any radio, and of many other types of electronic circuits; the mixer has a major influence upon the overall performance of the system.
Existing mixer architectures appearing in the prior art (market, professional papers, patent filings) are generally combinations of compromises that require the designer to deal with various permutations of cost, size, reliability, performance, complexity, power dissipation, and other potentially problematic issues. Worse, imperfect mixer performance multiplies problems in subsequent circuitry. The mixer, therefore, is fundamentally important in radio frequency circuit design, and can have a profound effect upon overall system merit.
A mixer design is characterized by cost, size, and power dissipation, and also by these performance specifications:
CONVERSION LOSS is a measure of the efficiency of the mixer in providing frequency translation from the input signal to the output signal. Conversion loss of a mixer is equal to the ratio of the IF single sideband output power to the RF input power, expressed as a positive number in dB. The lower the loss, the more efficient the mixer. In many designs, one or another of the mixer's inputs and/or outputs are amplified within the overall mixer circuit, thus enabling the management of conversion loss and even providing conversion GAIN, but amplification by its nature introduces noise and other artifacts.
CONVERSION COMPRESSION is a measure of the maximum RF input signal for which the mixer will provide linear operation in terms of constant conversion loss. This specification enables the comparison of dynamic range for various mixers, and the maximum input power.
ISOLATION is a measure of the circuit balance within the mixer. When isolation is high, the “leakage” or “feed through” between mixer ports will be small, and the inverse is true. Typically, mixer isolation falls off with frequency due to the imbalance of any transformer, lead inductance, and capacitive imbalance between mixer circuit components such as diodes.
DYNAMIC RANGE is the signal power range over which a given mixer design operates effectively without conversion compression. The conversion compression point identifies the upper limit of dynamic range; the NOISE FIGURE, the BANDWIDTH, and the level of INTERMODULATION PRODUCTS of the mixer circuit identify the lower limit of dynamic range.
INTERMODULATION distortion takes place when two RF signals simultaneously enter the mixer non-linear RF port and interact to produce modulation of either signal by the other, resulting in undesired signal artifacts. This can occur in a multiple-carrier signal environment, or when an undesired signal interferes with a desired one. Also, an imperfect mixer generates its own intermodulation distortion due to its non-linearity. The products resulting from the interaction are usually objectionable, and impose limits upon the design of the overall circuit when they fall within the frequency range of the mixer output.
INTERCEPT POINT is a commonly accepted and useful method of describing the capability of a mixer to suppress two-tone, third order intermodulation distortion, using the “third-order intercept” approach. The third-order intercept point (IP3) is a theoretical location on the output versus input line where the desired output signal (each of the two tones) and the two third-order products (each one) become equal in power, as RF input power is raised. This single mixer specification usually defines the overall performance of the mixer design, and its utility in the circuit.
The ideal frequency mixer is essentially linear, with output spectra that include fewer artifacts and noise than less ideal designs. In such an optimized design the mixing function generates fewer products or artifacts from the input and LO signals. In conventional mixer designs, LO current does flow through the device that accomplishes the mixing, and that device is typically nonlinear.
A theoretically ideal mixer will generate the desired output with no artifacts and noise, and most specifically no output energy resulting from LO currents flowing through the mixer device.
Even in an idealized mixer, there can be some products or signal artifacts due to angle cuttings. However, compared to mixers in the prior art, the idealized mixer will generate these products at a significantly lower power level, and their effect on overall system performance will be less. Mixers in the prior art are defined by the specification called the 3rd intercept point (IP3), and conventional mixers achieve an IP3 level at about 20 dBm. This single specification defines mixer performance.
The current invention is a mixer architecture that uses linear devices such as field effect transistors (FETs) as LO-controlled variable resistors, or actual variable resistors, or another linear variable device, and avoids LO current within the mixer circuit, thus achieving significantly higher IP3 numbers. This parameter supports very aggressive circuit designs not previously possible, and overall system performance not previously achievable.
1. Prior Art
In the professional literature, commercial market, and in the files of various patent offices, there exist hundreds of different mixer designs. All seek to produce a commercially viable combination of linearity, cost, conversion loss, reliability, and similar factors, but none use the architecture of the present invention, and none provide performance of the parallel channel mixing circuit used in an embodiment of the present invention. All designs that use nonlinear devices produce an IP3 parameter that is well below the performance of the present invention.
Many excellent mixer configurations have been developed. However, evolving systems can benefit from clarity that is not possible with nonlinear devices, and the inherent problems of conventional mixers put an ever-increasing burden on the circuit designer, requiring compromises in critical areas. Ordinary design compromises are generally not necessary when the present invention is used.
2. Objectives of the Present Invention
The present invention provides circuit designers with a cost-effective mixer that has an IP3 (third order intercept point), and also higher order IPs, substantially improved over the capabilities of conventional mixer technologies.
Brief Summary of the Present Invention
The present invention is a mixer or frequency converter that uses field effect transistors, resistors, or other linear devices/circuits as controllable mixing devices, in circuits that prevent local oscillator (LO) currents from appearing in the mixer, thus providing features, functions, and performance not achieved by conventional designs.
It will be recognized that some or all of the Figures are schematic representations for purposes of illustration and do not necessarily depict the actual relative sizes or locations of the elements shown. Unless otherwise specifically noted, articles depicted in the drawings are not necessarily drawn to scale. The Figures are provided for the purpose of illustrating one or more embodiments of the invention with the explicit understanding that they will not be used to limit the scope or the meaning of the claims.